1. Field of the Invention
The present invention relates to an electron gun for a cathode-ray tube and, more particularly, to a precision electron gun improved by the reduction of deformation of an electrode during assembly as well as to a method of assembling such an electrode gun.
2. Description of the Prior Art
Cathode-ray tubes, such as color picture tubes or monitor tubes, are composed to cause electron beams emitted from an electron gun accommodated at one end of a vacuum envelope to strike and excite a phosphor screen formed at the other end thereof, thereby reproducing an image according to the intensity of the electron beams modulated with an image signal.
FIG. 6 is an explanatory cross-sectional view of the structure of this kind of cathode-ray tube. The cathode-ray tube shown in FIG. 6 includes a faceplate 21, a funnel 22, a neck 23 contiguous to the funnel 22, an electron gun 24, a shadow mask 25 having a multiplicity of electron-beam passage holes (apertures), a magnetic shield 26, a phosphor screen 27, a deflection yoke 28, electron beams 29, a magnetism correcting magnet 30 for purity adjustment and the like, a getter 31, and a junction 32 at which the faceplate 21 and the funnel 22 are joined together.
As shown in FIG. 6, the faceplate 21, the funnel 22 and the neck 23 constitute a vacuum envelope, and the electron gun 24 is accommodated in the neck 23. The shadow mask 25, having a multiplicity of apertures and opposed to the phosphor screen 27 at a predetermined distance, is suspended from a skirt portion inside of the faceplate 21.
Although not shown, a so-called exterior conductor film is applied to the outside of the funnel 22, and the inside wall of the funnel 22 has a uniformly applied conductor film of graphite or the like which extends into part of the neck 23. The funnel 22 is also provided with an anode terminal which is disposed to extend through the conductor film and the exterior conductor film respectively applied to the inside and the outside of the funnel 22 as well as a portion of the funnel 22. The electron beams 29 are deflected horizontally and vertically by the deflection yoke 28 mounted on the outside wall of the portion of the vacuum envelope in which the funnel 22 meets the neck 23, thereby two-dimensionally scanning the phosphor screen 27.
The phosphor screen 27 is coated with three color phosphors for red, green and blue in stripes or dots, and the three electron beams 29 emitted from the electron gun 24 are selected by the shadow mask 25 and strike the respective color phosphors to make them emit light.
The electron gun 24 includes a plurality of coaxial electrodes, such as cathodes for generating electron currents, a control electrode, an accelerating electrode and a focusing electrode, and these electrodes are fixed and held at predetermined intervals by an insulating material (beading glass), such as bead glass.
FIG. 7a is an explanatory view of the assembly of electron gun electrodes according to a prior art, more specifically, an explanatory diagrammatic view of the assembly of a composite electrode which includes two electrode elements joined together in opposed relationship to each other.
FIG. 7a shows the assembly of the composite electrode which includes the two electrode elements joined together at their flanges. In FIG. 7a, reference numeral 41 denotes a composite electrode which includes electrode elements 4-1 and 4-2, reference numeral 4-1a denotes a flange of the electrode element 4-1, reference numeral 4-2a denotes a flange of the electrode element 4-2, reference numeral 10 denotes a spacer, reference numeral 11 denotes an alignment core of assembling tools, and reference numerals 12 and 13 denote the respective assembling tools. The respective electrode elements 4-1 and 4-2, which constitute the composite electrode 41 which is made up of two electrode elements to serve as one electrode, have the flanges 4-1a and 4-2a formed to be buried in the beading glass. The two electrode elements 4-1 and 4-2 are fitted onto the alignment core 11 of the assembling tools 12 and 13 in a stacked manner with the flanges 4-1a and 4-2a being opposed to each other, and the flanges 4-1a and 4-2a are united together, as by laser welding, within a predetermined space restricted by the spacer 10, thus preparing the composite electrode 41. The flanges 4-1a and 4-2a together with the other electrodes are fixedly buried in the beading glass.
FIG. 7b is an explanatory view of the assembly of a composite electrode which includes two electrode elements joined together at their cup-shaped bottom portions. In FIG. 7b, reference numeral 41' denotes a composite electrode which includes electrode elements 4'-1 and 4'-2, reference numeral 4'-1a denotes a flange of the electrode element 4'-1, reference numeral 4'-2a denotes a flange of the electrode element 4'-2, reference numeral 4'-1b denotes a cup-shaped bottom portion of the electrode element 4'-1, and reference numeral 4'-2b denotes a cup-shaped bottom portion of the electrode element 4'-2. Reference numerals 10, 11, 12 and 13 respectively denote a spacer, an alignment core of assembling tools, and the assembling tools, all of which are identical to those shown in FIG. 7a. Similarly to the prior art shown in FIG. 7a, the respective electrode elements 4'-1 and 4'-2, which constitute the composite electrode 4', which is made up of two electrode elements to serve as one electrode, have the flanges 4'-1a and 4'-2a formed to be buried in the beading glass. The two electrode elements 4'-1 and 4'-2 are fitted onto the alignment core 11 of the assembling tools 12 and 13 in a stacked manner with the cup-shaped portions 4'-1b and 4'-2b being opposed to each other, and the cup-shaped portions 4'-1b and 4'-2b are united together, as by laser welding, within a predetermined space restricted by the spacer 10, thus preparing the composite electrode 41'. The flanges 4'-1a and 4'-2a together with the other electrodes are fixedly buried in the beading glass.
FIG. 8a is an explanatory view of one of the above-described electrode elements, FIG. 8b is a cross-sectional view taken along line C-O-C, of FIG. 8a, FIGS. 8c and 8d are explanatory enlarged views showing deformation of the portion D shown in FIG. 8b, and FIGS. 8e and 8f are explanatory enlarged views showing deformation of the portion E shown in FIG. 8b.
In FIGS. 8a to 8f, the reference numerals marked with a prime indicate the constituent portions of the composite electrode shown in FIG. 7b which includes the two electrode elements joined together at their cup-shaped bottom portions, while the other reference numerals indicate the constituent portions of the composite electrode shown in FIG. 7a which includes the two electrode elements joined together at their flanges.
In the process of forming the composite electrode by joining the electrode elements together at their flanges as shown in FIG. 7a, the flange 4-2a is press-formed integrally with the electrode element 4-2 in parallel with a plane normal to the axial direction of the electrode element 4-2. In practice, however, as shown in FIGS. 8c and 8d, the flange 4-2a tends to be curved as indicated by .theta..sub.1 or .theta..sub.2 toward or away from the other electrode element owing to bending stress. If such two electrode elements are united together, it will be difficult to accurately obtain the electrode height H shown in FIG. 7a.
In the process of forming the composite electrode by joining the electrode elements together at their cup-shaped bottom portions as shown in FIG. 7b, the bottom portion 4'-1b of the electrode element 4'-1 to be united to the bottom portion of the other electrode element tends to be curved as indicated by .theta..sub.3 or .theta..sub.4 away from or toward the other electrode element, as shown in FIGS. 8e and 8f, owing to bending stress similar to the above-described one. If such two electrode elements are united together, it will be difficult to accurately obtain the electrode height H' shown in FIG. 7b.
As described above, electron gun assemblies for use in color cathode-ray tubes or the like are assembled by stacking a plurality of electrodes including the above-described composite electrode by means of assembling tools and insulatively holding the stacked electrodes at predetermined intervals with beading glass.
The assembly precision of such electron gun is determined by the concentricity of the electron-beam passage holes of the respective electrodes and the degree of squareness between the end face of each of the electrodes and the axis of the electron gun, and this precision greatly influences the focusing characteristics of the cathode-ray tubes.
A prior art which relates to this kind of electron gun is disclosed in, for example, Japanese Patent Laid-Open No. 136134/1985.
As is apparent from the above description, the aforesaid prior art involves the problem that deformation due to bending stress occurs in the flange portion or the cup-shaped bottom portion of either of the electrode elements of the composite electrode which constitutes part of the electron gun, and this leads to a lowering in the assembly precision of a finished electron gun.